A perfectly balanced air and fuel mixture is commonly known as a ‘stoichiometric’ mixture. Theoretically, this type of mixture will burn all of the available fuel charge using all of the oxygen available in the air charge. The normalized fuel/air equivalence ratio, designated φ, or phi, is defined as the actual fuel/air ratio divided by the stoichiometric fuel/air ratio for any particular fuel of interest in any consistent set of units (mass ratio, volume ratio, etc.). The fuel/air ratio φ, or phi, is also equivalent to the inverse of the normalized fuel/air ratio known as lambda, or λ. φ equals 1 for a stoichiometric fuel/air ratio. When excess fuel is present (φ greater than one), the mixture is referred to as a ‘rich’ mixture. On the other hand, when excess air is present (φ less than one), the mixture is referred to as a ‘lean’ mixture.
A problem with the use of hydrogen as a motor fuel occurs due to the high flame speed of hydrogen. Hydrogen has an extremely rapid flame speed near stoichiometric fuel/air ratio conditions, which leads to unstable combustion, high oxides of nitrogen (NOx) emissions, and an audible “knocking” sound. The knocking sounds occur even when the combustion is not in a state of true knock from preignition or detonation. For these reasons, most hydrogen-fueled internal combustion engines have only been operated at lean fuel/air ratios.
To slow the hydrogen flame speed for stable combustion, fuel/air equivalence ratios of approximately 0.8φ or less are necessary, and acceptable NOx emissions may require equivalence ratios of 0.5φ or less. The wide flammability range of hydrogen allows operation down to equivalence ratios of about 0.2φ. These lean fuel/air ratio conditions limit the specific power output of hydrogen engines, so supercharging or turbo charging is often used to bring the output back up to typical diesel or gasoline power levels.
For the lowest possible exhaust emissions, it is generally desirable to operate spark-ignited internal combustion engines at a stoichiometric fuel/air ratio. This allows the use of a three-way exhaust catalyst, which oxidizes any unburned fuel and carbon monoxide, and reduces almost all of the NOx emissions and any unreacted oxygen. Emissions with a three-way (HC+CO+NOx) catalyst are much less than lean-burn emissions, especially regarding NOx. Current catalyst technology cannot effectively reduce NOx emissions of lean-burn engines. The extremely lean operating conditions possible with hydrogen fuel can dramatically reduce NOx emissions, but only at the expense of specific power and efficiency.
Lean burning engines avoid high NOx emissions by diluting the charge with extra air to reduce peak flame temperature. This dilution also reduces flame speed, which is excessive for near-stoichiometric hydrogen/air mixtures. It is also possible to dilute the charge with exhaust gas. By using exhaust gas recirculation (EGR), with stoichiometric overall fuel/air ratio, the dilution effects are similar to air dilution. The flame speed and combustion temperatures are reduced to stabilize the combustion and further decrease NOx formation. The EGR diluted, stoichiometric exhaust lacks excess oxygen to interfere with the NOx-reduction activity of the three-way catalysts, and less oxygen is available in the combustion chamber for NOx formation, relative to lean-burn air dilution. Any unburned hydrogen is a powerful reducing agent for efficient catalysis of nitrogen oxides at low exhaust temperatures.
In a typical spark ignited engine, there are efficiency losses due to throttling, known as pumping work. A throttle plate is used to control airflow through the intake manifold into the combustion chamber. It is advantageous to keep the pressure drop across the throttle to a minimum, whenever possible, to avoid efficiency losses due to pumping work. However, there are problems associated with the operation of lean-burn hydrogen engines at wide open throttle. At very low loads, throttling is typically needed to control the combustion stability of hydrogen. This also limits the unburned hydrogen emissions. At medium to high loads, the power output of a lean-burn hydrogen engine may be controlled by varying the fuel/air ratio at wide open throttle, but a reasonable power output for a given engine size requires fuel/air ratios rich enough to substantially increase the NOx emissions.
The methods of EGR can be broadly classified as either ‘internal’ or ‘external’. By closing the exhaust valve early or leaving it open through the intake valve opening and the beginning of the intake stroke, exhaust gases can either be trapped or drawn back into the engine cylinder for internal EGR. With a naturally aspirated hydrogen engine, a fixed-timing cam with delayed exhaust valve closing can be designed to deliver an appropriate EGR percentage at wide open throttle. At lower loads, as the air intake throttle is closed, the intake manifold pressure is reduced relative to the exhaust manifold pressure. This reduces power by both throttling the intake air charge and by drawing in additional EGR. However, the amount of throttling at a given part-load condition is greatly reduced, which also reduces pumping losses and improves part-load efficiency. Internal EGR may also be controlled with variable valve timing control, common on many modern gasoline engines. With variable valve timing, the EGR percentage can be directly controlled between the exhaust valve closing and intake valve opening times, and no external throttling is necessary.
Internal EGR is hot EGR, which may increase the likelihood of preignition and/or intake backfiring. Hydrogen fuel has a fairly high autoignition temperature, but the ignition energy requirement is extremely low. Therefore, hot exhaust gases, particles, or residual combustion reactions may ignite the incoming air/hydrogen mixture. These risks can be mitigated by combustion chamber design details and good lubrication oil control. In addition, the simplicity of the fixed-cam-timing, internal EGR system may outweigh the small efficiency losses resulting from slight throttling, compared to almost completely unthrottled, variable valve timing or external EGR systems. On the other hand, designing a fixed-timing cam for turbocharged engines may be difficult, due to the uncertain relationship between the intake and exhaust manifold pressures at various operating conditions. Internal EGR with fixed cam timing may also require delayed intake valve opening and near-zero valve timing overlap for supercharged engines, since the intake pressure is almost always greater than the exhaust pressure. In this case, additional part load throttling may also be required.
In contrast to internal EGR control systems, external EGR is plumbed from the exhaust manifold, or further downstream, back to the intake manifold, or further upstream, through a separate metering valve or flow control device. This external EGR flow can be further categorized as ‘hot’ or ‘cooled’. For conventional gasoline engines utilizing hot EGR, the EGR flow is relatively small, so the passive cooling provided by the EGR plumbing and mixing with cooler intake air is sufficient to avoid any problems due to excessive intake temperature. However, modern diesel engines utilizing very high EGR flow rates use a dedicated EGR heat exchanger, cooled by the engine coolant loop. Hydrogen engines with EGR for torque control can have EGR flow rates even higher than typical diesels, and they can utilize the EGR coolers developed for similar or somewhat larger diesel engines.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.